[AC Question]

I'm looking to buy a new phone but I dont know much about the Android phones available at the moment. I'd really appreciate some guidance..
These are my requirements.
Small.
Long battery life.
Removable battery.
Can accept a 128gb microsd card.
Can use 4g.
Can debug on via usb.

[Rukbat]

All current phones (except some cheap Chinese clones) can use 4G and can use USB debugging. Long battery life is a problem with Android phones - iPhones get better life. You can always carry a power bank if you need more life. Removable batteries and external SD cards seem to be going the way of the Dodo bird. I don't know what you mean by small - under 5"? Under 4"?

There really aren't many choices in new phones that meet all your requirements (any phone rated to use a 64GB SD card will be able to use a 128GB SD card), so buying an older phone (S4? S3?) might be an option.